The present invention relates to a flash control system applicable to a camera. More specifically, the present invention relates to a flash control system which is capable of correcting exposure errors due to a difference of reflectivity of objects having different colors when an object is photographed using flash light.
Conventionally, a flash device, which enables to photograph at correct exposure by measuring the brightness of an object detecting flash light reflected from an object, has become widespread. In such a flash device, a time period for which the flash light is emitted is determined based on the measured brightness, and emission of flash light is controlled so that the amount of light exposure becomes a correct value. Further, in such a flash device, the reflectivity of an object is generally assumed to be 18% and the time period for which the flash light is emitted is determined on this assumption.
Therefore, regarding a whitish object whose reflectivity is greater than 18%, the brightness determined based on the flash light reflected from the object is greater than the actual brightness because the determined brightness assumes only 18% reflectivity contributing to the brightness, when the reflectivity contribution is actually higher. If a whitish object is exposed by a camera including such a flash device, the object is under exposed because a time period for which flash light is emitted becomes shorter than a proper time period.
A dark object whose reflectivity is less than 18% is measured to have a lower brightness than the actual brightness because the measured brightness assumes 18% reflectivity contributing to the brightness, when the reflectivity is actually lower. Therefore, such an object is over exposed.
The difference of the reflectivity of objects may also occur depending on the color of objects. For example, when the color of an object is yellow, the reflectivity may be up to 70%. In such a case, if the standard reflectivity is assumed to be 18%, the exposure value is approximately 2 Ev lower than necessary. If the object color is blue, the reflectivity is approximately 9%. In this case, the object is over exposed by approximately 1 Ev greater than necessary.
Therefore, in general, a camera including such a flash device is configured such that exposure parameters (e.g., the time period for which flash light is emitted) is adjusted by a photographer. For example, the photographer guesses the reflectivity of the object and adjusts the exposure parameters so that if the object is a whitish or yellowish one having a high reflectivity, it is to be overexposed, and if the object is a blackish or bluish one having a low reflectivity, it is to be underexposed.
With this operation, the above-described defects may be solved. However, accurately guessing the reflectivity of the object and adjusting the exposure parameters can only be done by experienced and skilled photographers. It is impossible to require all the photographers to do such an operation. Further, it is not preferable that a manual operation of the photographer is required for exposure. Furthermore, if such a manual operation is required, cameras become unsuitable for automatic photographing which is the recent trend.
Recently, a flash control system having a function of measuring the color of the object has been provided. In such a flash control system, the reflectivity of the object is determined based on the measured color, and the time period for which flash light is emitted is controlled based on the determined reflectivity of the object. Since the flash control system obtains accurate information about the reflectivity of the object, the flash control system has an advantage in its suitability for photographing the object at correct exposure using flash light.
However, the flash control system measures the color of the object utilizing natural light, such as sunlight, reflected from the object. Therefore, there may be cases where a color of the object determined by the flash control system utilizing natural light does not match with a color of the object when flash light Is emitted. That is because there is a difference between color rendering properties of natural light and flash light.
The above-described problem occurs when a color of an object is determined using various types of flash light having different color rendering properties.
The present invention is advantageous in that it provides a flash control system which enables to photograph objects at correct exposure using flash light.
According to an aspect of the invention, there is provided a flash control system for a camera, which is provided with an automatic flash device that controls an amount of emission of flash light based on a result of a comparison between an amount of light sensed by a photometric sensor for normal light and a predetermined appropriate amount of light, a control system that preliminarily emits the flash light, and first photometric sensors for colorimetry capable of performing colorimetry. The first photometric sensors for colorimetry sense different components of light, respectively. The flash control system is further provided with a first color determining system that determines a color of an object based on outputs of the first photometric sensors for colorimetry. The outputs of the first photometric sensors for colorimetry is obtained when the control system preliminarily emits the flash light. Further, the flash control system is provided with a first colorimetric compensation value determining system that determines a first colorimetric compensation value based on the color determined by the first color determining system, and a flash control level adjusting system that adjusts the predetermined appropriate amount of light based on the first colorimetric compensation value. In this case, the automatic flash device uses the predetermined appropriate amount of light adjusted by the flash control level adjusting system to control the amount of emission of flash light.
With this configuration, since the predetermined appropriate amount of light is adjusted based on the color of the object, it becomes possible to photographing the object using the flash light at correct exposure regardless of the color difference of objects (i.e., a difference of reflectivity of objects).
Optionally, each of the first photometric sensors for colorimetry may have a plurality of photometry areas, and may be capable of performing photometry with respect to each of the plurality of photometry areas. In this case, the color determining system may determine the color of the object for each of the plurality of photometry areas. Further, the first colorimetric compensation value determining system may determine the first colorimetric compensation value at each of the plurality of photometry areas, and may determine a second colorimetric compensation value based on the first colorimetric value determined at each of the plurality of photometry areas. The flash control level adjusting system may adjust the predetermined appropriate amount of light according to the second colorimetric compensation value.
In a particular case, the first colorimetric compensation value determining system may determine the second colorimetric compensation value as an average, a center-weighted average, or a maximum value of the first compensation value determined at each of the plurality of photometry areas.
Optionally, the first color determining system may compensate for the outputs of the first photometric sensors for colorimetry depending on data intrinsic to a photographing lens that forms an image of the object, and may determine the color of the object based on the compensated outputs of the first photometric sensors for colorimetry.
Still optionally, the first color determining system may compensate for the outputs of the first photometric sensors for colorimetry according to brightness data with regard to the flash light corresponding to a color rendering property of the flash light, and may determine the color of the object based on the compensated outputs of the first photometric sensors for colorimetry.
In a particular case, the automatic flash device may include an integrating circuit that outputs a quenching signal when the amount of light sensed by the photometric sensor reaches the predetermined appropriate amount of light adjusted by the flash control level adjusting system, and a light emission circuit that stops emission of the flash light when the quenching signal is output by the integrating circuit.
In a particular case, the predetermined appropriate amount of light may be determined based on at least film speed and the first colorimetric compensation value.
Optionally, the flash control system may include a photometry device that determines an exposure value based on normal light reflected from the object, and judges whether emission of the flash light is necessary for photographing the object based on the determined exposure value.
In a particular case, the photometry device may include a normal light sensor capable of performing photometry operation with respect to the object at each of the plurality of photometry areas. The normal light sensor may have a spectral sensitivity characteristic close to those of human eye. The photometry device may further include second photometric sensors for colorimetry capable of performing colorimetry with respect to each of the photometry areas. The second photometric sensors for colorimetry sense different components of light, respectively. Further, the photometry device may include a photometry value determining system that determines an photometry value at each of the plurality of photometry areas in accordance with outputs of the normal light sensor corresponding to the plurality of photometry areas, and a second color determining system that compensates for outputs of the second photometric sensors for colorimetry according to brightness data with regard to the normal light corresponding to a color rendering property of the normal light, and determines a color of the object at each of the plurality of photometry areas based on the compensated outputs of the second photometric sensors for colorimetry. Further, the photometry device may include a second colorimetric compensation value determining system that determines a second colorimetric compensation value at each of the plularity of photometry areas based on the color determined by the second color determining system, and an exposure value determining system that compensates for the photometry value determined by the photometry value determining system for each of the plurality of photometry areas according to the second colorimetric compensation value, and determines the exposure value based on the compensated photometry value. In this case, the predetermined appropriate amount of light is determined based on the exposure value.
In a particular case, the first photometric sensors for colorimetry may be used as the second photometric sensors for colorimetry.
Optionally, the flash control system may include a light source sensor capable of performing colorimetry. The light source sensor may have photometry areas whose spectral sensitivity characteristics are equal to those of the second photometry sensors, respectively. In this case, the second color determining system may compensate for the outputs of the second photometric sensors for colorimetry according to an output of the light source sensor in place of the brightness data with regard to the normal light.
In a particular case, the first photometric sensors for colorimetry may include three sensors which sense a green color component, a red color component, and a blue color component of light, respectively.
According to another aspect of the invention, there is provided a flash control system for a camera having a light sensor that obtains a photometry value, which is provided with photometric sensors for colorimetry, a light source sensor capable of obtaining brightness data corresponding to a color rendering property of normal light. The flash control system further provided with a first compensator that compensates for the photometry value obtained by the light sensor according to a color of an object when photographing is performed using the normal light. The color of the object is determined based on outputs of the photometric sensors for colorimetry and the brightness data. Further, the flash control system is provided with a second compensator that compensates for a predetermined appropriate amount of light according to the color of the object when photographing is performed using flash light. The color of the object is determined based on the outputs of the photometric sensors for colorimetry when the flash light is preliminarily emitted and a predetermined compensation value corresponding to a color rendering property of the flash light.